17,909 research outputs found

### Dilaton Black Holes Near the Horizon

Generic $U(1)^2$ 4-d black holes with unbroken $N=1$ supersymmetry are shown
to tend to a Robinson-Bertotti type geometry with a linear dilaton and doubling
of unbroken supersymmetries near the horizon. Purely magnetic dilatonic black
holes, which have unbroken $N=2$ supersymmetry, behave near the horizon as a
2-d linear dilaton vacuum $\otimes \, S^2$. This geometry is invariant under 8
supersymmetries, i.e. half of the original $N=4$ supersymmetries are unbroken.
The supersymmetric positivity bound, which requires the mass of the 4-d dilaton
black holes to be greater than or equal to the central charge, corresponds to
positivity of mass for a class of stringy 2-d black holes.Comment: 10 pages, SU-ITP-92-2

### Localized Activation of Bending in Proximal, Medial and Distal Regions of Sea-Urchin Sperm Flagella

Spermatozoa from the sea urchin, Colobocentrotus atratus, were partially demembranated by extraction with solutions containing Triton X-100 at a concentration which was insufficient to solubilize the membranes completely. The resulting suspension was a mixture containing some spermatozoa in which a proximal, medial, or distal portion of the flagellum was membrane-covered, while the remaining portion was naked axoneme. In reactivating solutions containing 12 µM ATP, only the naked portions of the flagellum became motile. In reactivating solutions containing 0.8 mM ADP, the membrane-covered regions became motile and beat at 6-10 beats/s, while the naked regions remained immobile, or beat very slowly at about 0.3 beat/s. Activation of membrane-covered regions in ADP solutions probably results from the membrane restricting the diffusion of ATP which is formed from ADP by the axonemal adenylate kinase. The results indicate that any region of the flagellum has the capacity for autonomous beating, and that special properties of the basal end of the flagellum are not required for bend initiation. However, the beating of different regions of the flagellum is not completely independent, for in a fair number of spermatozoa the beating of the distal, membrane-covered region in 0.8 mM ADP was intermittent, and was turned on and off in phase with the much slower bending cycle in the proximal region of naked axoneme

### 3-Branes and Uniqueness of the Salam-Sezgin Vacuum

We prove the uniqueness of the supersymmetric Salam-Sezgin
(Minkowski)_4\times S^2 ground state among all nonsingular solutions with a
four-dimensional Poincare, de Sitter or anti-de Sitter symmetry. We construct
the most general solutions with an axial symmetry in the two-dimensional
internal space, and show that included amongst these is a family that is
non-singular away from a conical defect at one pole of a distorted 2-sphere.
These solutions admit the interpretation of 3-branes with negative tension.Comment: Latex, 12 pages; typos corrected, discussion of brane tensions
amende

### Generalized dilaton-Maxwell cosmic string and wall solutions

The class of static solutions found by Gibbons and Wells for
dilaton-electrodynamics in flat spacetime, which describe nontopological
strings and walls that trap magnetic flux, is extended to a class of dynamical
solutions supporting arbitrarily large, nondissipative traveling waves, using
techniques previously applied to global and local topological defects. These
solutions can then be used in conjunction with S-duality to obtain more general
solitonic solutions for various axidilaton-Maxwell theories. As an example, a
set of dynamical solutions is found for axion, dilaton, and Maxwell fields in
low energy heterotic string theory using the SL(2,R) invariance of the
equations of motion.Comment: 11 pages; to appear in Phys.Lett.

### Topology, Entropy and Witten Index of Dilaton Black Holes

We have found that for extreme dilaton black holes an inner boundary must be
introduced in addition to the outer boundary to give an integer value to the
Euler number. The resulting manifolds have (if one identifies imaginary time)
topology $S^1 \times R \times S^2$ and Euler number $\chi = 0$ in contrast to
the non-extreme case with $\chi=2$. The entropy of extreme $U(1)$ dilaton black
holes is already known to be zero. We include a review of some recent ideas due
to Hawking on the Reissner-Nordstr\"om case. By regarding all extreme black
holes as having an inner boundary, we conclude that the entropy of {\sl all}
extreme black holes, including $[U(1)]^2$ black holes, vanishes. We discuss the
relevance of this to the vanishing of quantum corrections and the idea that the
functional integral for extreme holes gives a Witten Index. We have studied
also the topology of ``moduli space'' of multi black holes. The quantum
mechanics on black hole moduli spaces is expected to be supersymmetric despite
the fact that they are not HyperK\"ahler since the corresponding geometry has
torsion unlike the BPS monopole case. Finally, we describe the possibility of
extreme black hole fission for states with an energy gap. The energy released,
as a proportion of the initial rest mass, during the decay of an
electro-magnetic black hole is 300 times greater than that released by the
fission of an ${}^{235} U$ nucleus.Comment: 51 pages, 4 figures, LaTeX. Considerably extended version. New
sections include discussion of the Witten index, topology of the moduli
space, black hole sigma model, and black hole fission with huge energy
releas

### Discrete Newtonian Cosmology

In this paper we lay down the foundations for a purely Newtonian theory of
cosmology, valid at scales small compared with the Hubble radius, using only
Newtonian point particles acted on by gravity and a possible cosmological term.
We describe the cosmological background which is given by an exact solution of
the equations of motion in which the particles expand homothetically with their
comoving positions constituting a central configuration. We point out, using
previous work, that an important class of central configurations are
homogeneous and isotropic, thus justifying the usual assumptions of elementary
treatments. The scale factor is shown to satisfy the standard Raychaudhuri and
Friedmann equations without making any fluid dynamic or continuum
approximations. Since we make no commitment as to the identity of the point
particles, our results are valid for cold dark matter, galaxies, or clusters of
galaxies. In future publications we plan to discuss perturbations of our
cosmological background from the point particle viewpoint laid down in this
paper and show consistency with much standard theory usually obtained by more
complicated and conceptually less clear continuum methods. Apart from its
potential use in large scale structure studies, we believe that out approach
has great pedagogic advantages over existing elementary treatments of the
expanding universe, since it requires no use of general relativity or continuum
mechanics but concentrates on the basic physics: Newton's laws for
gravitationally interacting particles.Comment: 33 pages; typos fixed, references added, some clarification

### Some Comments on Gravitational Entropy and the Inverse Mean Curvature Flow

The Geroch-Wald-Jang-Huisken-Ilmanen approach to the positive energy problem
to may be extended to give a negative lower bound for the mass of
asymptotically Anti-de-Sitter spacetimes containing horizons with exotic
topologies having ends or infinities of the form $\Sigma_g \times {\Bbb R}$, in
terms of the cosmological constant. We also show how the method gives a lower
bound for for the mass of time-symmetric initial data sets for black holes with
vectors and scalars in terms of the mass, $|Z(Q,P)|$ of the double extreme
black hole with the same charges. I also give a lower bound for the area of an
apparent horizon, and hence a lower bound for the entropy in terms of the same
function $|Z(Q,P)|$. This shows that the so-called attractor behaviour extends
beyond the static spherically symmetric case. and underscores the general
importance of the function $|Z(Q,P)|$. There are hints that higher dimensional
generalizations may involve the Yamabe conjectures.Comment: 13pp. late

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